Breakdown Characteristics Of Insulation Research Articles

AC-DC breakdown characteristics of oil-paper insulation at low temperature

New energy sources such as wind energy are mainly distributed in low temperature areas such as high altitude and high dimensionality in inland China, which has higher requirements for the low-temperature insulation characteristics of power equipment. This poses higher demands for the low-temperature insulation capabilities of power equipment. However, the existing research on the low temperature breakdown characteristics of oil-impregnated paper is limited. In this study, the authors constructed an AC-DC combined power supply and a specialized low-temperature breakdown testing platform. Various oil-impregnated paper samples with different moisture contents were prepared. The paper investigated the AC-DC breakdown characteristics of these samples under low-temperature conditions, focusing on the effects of temperature, moisture content, and DC component on the breakdown strength. The findings serve as a valuable design reference for the application of oil-impregnated power equipment in low-temperature environments.

Effects of Pre-Crosslinking on Space Charge and Breakdown Characteristics of XLPE Cable Insulation

This study focused on the effect of pre-crosslinking on the electrical properties of XLPE insulation for HVDC cables. The XLPE samples were pre-crosslinked at 140, 150 and 160 °C. The space charge and DC breakdown characteristics of the XLPE samples with and without pre-crosslinking were investigated. The results show that the pre-crosslinked XLPE samples have an obvious space charge injection and transportation, resulting in more severe electric field distortion. Under the electric field of 100 kV/mm, the electric field distortion rates of the XLPE samples pre-crosslinked at 150 and 160 °C reached 51% and 72% respectively, which were much higher than that of the XLPE sample without pre-crosslinking (14%.) Compared with the XLPE without pre-crosslinking, the breakdown strengths of XLPE pre-crosslinked at 140, 150 and 160 °C were reduced by 11%, 6% and 7% respectively, at the temperature of 50 °C. It is concluded that the pre-crosslinking leads to imperfect crystallization and micro-defects in the amorphous region of XLPE, thereby leading to more space charge accumulation and reduced DC breakdown. The pre-crosslinking of XLPE insulation therefore should be prevented during HVDC cable manufacturing.

Breakdown Characteristics of Composite Insulation for High Temperature Superconducting Cable

Polypropylene laminated paper (PPLP) and Kraft paper have been studied as multi-layer insulation for high temperature superconducting cable. The experiment was examined with AC and impulse voltages. Short-term and long-term breakdown of insulation samples were investigated. It was found from the experimental results that the breakdown voltage becomes smaller in the order of PPLP, PPLP/Kraft and Kraft, but the lifetime index increases with the presence of Kraft layer in the multi-layer insulation. In addition, the position of PPLP layers has a significant effect on the breakdown voltage of PPLP/Kraft samples under positive impulse voltage. Finally, breakdown mechanism of PPLP/Kraft composite insulation was suggested. Partial discharges in butt-gaps and in the porous structure of insulation layers combined with electric discharges on the surface of these layers are considered to be main factors causing the puncture of multi-layer insulation.

Analysis and research on failure of B-phase lower bridge arm through wall casing in converter station

Based on the actual failure of the bushing, this paper conducts analysis and research. Firstly, it analyzes the protection action, on-site harmonic components, bushing structure and disassembly step by step. Then it conducts the research on the discharge and breakdown characteristics of epoxy insulation. Finally, we results show that there are bubbles or impurities in the outer capacitor layer of the capacitor core inside the casing, and the harmonic content is higher under the operating conditions of the converter station. Meanwhile, we gives some relevant suggestions for the through-wall bushing in the design and manufacturing stage to consider relevant harmonic factors are of certain significance to extend the life of the bushing.

Investigations of Particle-Initiated Insulation Breakdowns in Bearings

Early failures in bearings of wind turbine drivetrains have increased after introduction of power electronic switches, which leads to shaft voltages and bearing currents. In presence of voltage, a rupture of bearing insulation could occur due to several plausible electro-physical mechanisms viz., asperities, electric breakdowns, particles, etc. The flow of high amperage current through the bearing during a breakdown mechanism could lead to early failures. Our aim is to understand the electrical behaviour of a bearing and elaborate by an equivalent electric circuit model, emphasizing on particle-initiated breakdowns. In presence of a shaft voltage, the particles form a path of low resistance through the bearing and results in flow of shaft or bearing currents, which could cause pre-mature failure of the bearing. Particles such as Arizona Test Dust (ATD), carbon black, aluminium powder and fine iron powder were mixed in lubricant at particle concentrations ranging between 7.5 mg/L and 150 mg/L. The breakdown characteristics of electrical insulation of the bearing during a given test is quantified as time of conduction, which is expressed as a percentage of the time the bearing is in the conducting state during a test. An investigation of time of conduction for different lubricant samples was conducted along with studying the effects of start and stops of the rotating shaft. The electrical conductive nature of the particle played no role in breakdown of bearing voltage. At a fixed concentration of 150 mg/L, the insulation breakdown events were highest in lubricant with ATD, followed by iron powder, aluminium power and carbon black particles. The time of conduction increases up to 24 times for the same test lubricant, as the particle concentration was increased from 7.5 mg/L to 150 mg/L. The current activity reduced to almost half in the test after stopping the shaft rotation. The resistance of bearing during insulation breakdown events is highest for aluminium powder, followed by fine iron powder, carbon black and ATD.

Effect of magnetic field on partial discharge parameters in power cables and on the breakdown characteristics of impregnated paper insulation

This article demonstrates that parameters of partial discharges in power cables and impregnated paper insulation breakdown characteristics vary in presence of grid-frequency alternating magnetic field. Experimental and theoretical demonstrations confirm that magnetic field affects the probability of partial discharges in air cavities inside inhomogeneous insulation. Based on the developed method, the impact of magnetic field on breakdown probability and on Weibull distribution parameters that approximate experimental breakdown voltage data is estimated. The differences in chemical and structural properties of the breakdown area in electrical and electromagnetic fields are determined using scanning electron microscopy and energy dispersive spectroscopy.

Study on Insulation Breakdown Characteristics of Printed Circuit Board under Continuous Square Impulse Voltage

The widely distributed interconnects in printed circuit boards (PCBs) easily couple with high voltage under the action of electromagnetic pulses, which leads to insulation failure. In this study, the dielectric breakdown characteristics of four typical PCBs are studied under continuous square impulse voltage conditions. First, the electric field distribution in the four electrode models is simulated with the ANSYS software (ANSYS Maxwell 17.0). Electric field simulation results show the weak area of electric field distribution. On this basis, the possible breakdown patterns of PCB are analyzed. Second, the influence of factors, such as temperature, pulse duty ratio, interconnect insulation distance, and air pressure, on PCB breakdown voltage is studied through a breakdown test on the PCBs. Results show that the discharge between the single-layer electrodes of the PCBs is surface discharge, and the breakdown is that of a “gas–solid composite medium”. Meanwhile, the breakdown of a double-layer PCB is solid breakdown. Finally, scanning electron microscopy (SEM) produced by Tescan (Brno, Czech Republic) is performed to study the carbonization channel after PCB breakdown. SEM results reveal that the PCB carbonization channel is influenced by temperature and pressure in varying degrees.

Breakdown Characteristics of Oil-Pressboard Insulation under AC-DC Combined Voltage and Its Mathematical Model

An AC-DC combined voltage is applied to the oil-pressboard insulation near the valve side during the operation of a converter transformer. To study the breakdown characteristics of an oil-pressboard insulation under such voltages, a typical plate electrode structure was employed in the laboratory to conduct a breakdown test on the oil-pressboard insulation. The electrical field distribution and the DC contents of the transformer oil and the pressboard in composite insulation under the AC-DC combined voltage were simulated by their dielectric parameters. The breakdown strength of the transformer oil decreases with the increase in the DC content of the applied voltage, whereas that of the pressboard increases. For the oil-pressboard insulation, the breakdown voltage increases first and then decreases. The electric field strength decreases in the transformer oil with the increase in the DC content, whereas it increases in the pressboard. And the DC contents of the transformer and the pressboard in composite insulation were different from that of the applied voltage. Finally, based on the above results, a mathematical model was proposed to describe the breakdown characteristics of the oil-pressboard insulation under the AC-DC combined voltage; the theoretical and experimental results were in good agreement.

Research on breakdown characteristics of oil-paper insulation in compound field at different temperatures

The breakdown characteristics of oil-paper insulation in AC, DC and compound field at different temperatures were studied. The breakdown mechanism of oil-paper insulation at different temperatures and in AC and DC electric fields was analyzed. The breakdown characteristic mechanisms of the oil-paper insulation in the compound field at different temperatures were obtained: the dielectric strength of oil-paper compound insulation is changed gradually from dependence on oil dielectric strength to dependence on paperboard dielectric strength at low temperature. The dielectric strength of oil-paper compound insulation is always related to the oil dielectric strength closely at high temperature with decrease of AC content.

Research on breakdown characteristics of converter transformer oil-paper insulation under compound electric field in alpine region

In the paper, existing research and operating experience was summarized. On the basis, the particularity of oil-paper insulation operation condition for converter transformer was combined for studying the influence of temperature on oil-paper insulation field intensity distribution of converter transformers under different AC contents within wide temperature scope (-40°C∼105°C). The law of temperature gradients on space charge accumulation was analyzed. The breakdown or flashover characteristics of typical oil-paper compound insulation structure under the action of DC, AC and AC-DC superposition voltage at different temperatures were explored. The design principles of converter transformer oil-paper insulation structures in alpine region was proposed. The principle was adjusted and optimized properly according to the operation temperature scope and withstood AC-DC proportion. The reliability of transformer operation was improved on the one hand, and the insulating medium can be rationally utilized for reducing the manufacturing cost of the transformer on the other hand.

Electrical AC Insulation Breakdown Characteristics of Various Epoxy / Heterogeneous Inorganic Mixed Composite

Electrical AC Insulation Breakdown Characteristics of Various Epoxy / Heterogeneous Inorganic Mixed Composite

Effect of space charge on breakdown characteristics of aged oil-paper insulation under DC voltage

Due to its excellent performance, oil-paper insulation has been widely used in many power apparatuses including converter transformer of high voltage direct current (HVDC) system. Space charge accumulation phenomena may significantly distort local electric field inside transformer and cause oil-paper insulation overstressing. As a result of long term operation, oil-paper gradually degrades. In most severe cases, the breakdown of insulation system may occur. In this work, the effect of space charge on breakdown characteristics of aged oil-paper insulation is investigated. Oil-paper samples with different aging states are obtained by thermally accelerated aging for various periods of time, categorized by the degree of polymerization (DP) of 1107 (new), 725, 504, 376 and 241, respectively. Experimental results show that the breakdown voltage of oil-paper samples without pre-stressing remains almost constant with thermal degradation. After the homo-polarity pre-stress, when the DP value is higher than ∼300 (from 1107 to 376), the breakdown voltage has an increasing tendency with an increase in thermal degradation. On the contrary, the breakdown voltage presents a decreasing tendency after the hetero-polarity pre-stress. When the DP is lower than ∼300 (DP241), the changes of breakdown voltage does not remain the same trend as previous samples after pre-stressing. Meanwhile, space charge distributions in different aged samples are measured by the pulsed electro-acoustic (PEA) method, and the trap parameters are correspondingly evaluated. It has been experimentally demonstrated that special attention should be paid to space charge accumulation and aging state of oil-paper insulation, which may lead to severe insulation accidents.

Accumulative effect of repeated lightning impulses on transformer insulation: mechanism analysis

During the decades-long operation, oil paper insulation in power transformers may suffer the effect of thousands of lightning impulses. Such interesting but rarely studied phenomenon, also known as accumulative effect, may cause irreversible damage to the oil paper insulation, or even premature failure. Therefore, in this work, the influence of repeated lightning impulses on the oil paper insulation, especially the mechanism behind this phenomenon, was experimentally investigated. We determined the accumulative breakdown characteristics of the oil paper insulation and observed the influence of the repeated lightning impulses on the dissolved gas in oil and the degree of polymerization. Specifically, the space charge distribution in the oil paper samples and the relationship between the interval time and accumulative breakdown characteristics were experimentally determined in the end. The test results indicate that the accumulative effect only exists in the cellulose paper rather than the transformer oil. Such effect was considered to be the consequence of electrical degradation and accumulation of space charge.

Impact of time parameters of lightning impulse on the breakdown characteristics of oil paper insulation

At present, the insulation performances of electrical apparatus under lightning impulse voltages are always evaluated by the withstand test of standard lightning impulse voltage (1.2/50 µs), which is recommended by IEC Standard. However, the actual time parameters of transformer invading lightning impulse voltage present an obvious dispersibility characteristics. The standard wave shape is only representative of one particular class of lightning waves, which is far from satisfactory to act as the guidance waveform in the design of power equipment insulation. On the basis of the above research background, the relationship between breakdown characteristics of oil paper insulation and time parameters of applied lightning impulse voltage was investigated in this study. The 50% breakdown voltage characteristics of oil paper insulation under lightning impulse voltage with different time parameters were determined. The test results indicate that a short wave front time could lead to relatively low 50% breakdown voltage, while when the wave tail time varied, the 50% breakdown voltage of oil paper insulation barely changed. At last the simulation model of oil paper insulation was developed, and the current flowing through the oil paper insulating system under lightning impulse voltage was calculated. On the basis of the simulating results, the relationship between current and time parameters was analysed.

Breakdown characteristics of oil-paper insulation under lightning impulse waveforms with oscillations

The non-standard lightning impulse waveforms (N-SLIWs) including oscillating lightning impulses, very fast transient voltages (VFTOs) and steep waveforms in the power system gravely endanger the inner insulation of power transformer. To prevent insulation failure and provide guidance for robust transformer design, in this paper, two typical insulation models of oil-immersed transformer were established and their breakdown characteristics under N-SLIWs were investigated. The research mainly focused on the influence of front time and oscillation frequency of N-SLIWs on the breakdown characteristics. The experimental results showed that the breakdown voltage increased with the decrease of front time. For low frequency-oscillating lightning impulse waveforms (LF-OLIWs), breakdown voltage increased with the increase of oscillation frequency and was up to 30% higher than that under standard lightning impulse waveform (SLIW). For VFTOs, with the increase of frequency, breakdown voltages presented a law of inverted U type. Insulation model could withstand the highest value of VFTO when the oscillation frequency was around 2.4 MHz. But when the frequency was as high as 9 MHz, breakdown voltage was close to that under SLIW. A very high frequency oscillation was the main reason for the reduction of breakdown voltage as its steepness dV/dt assisted the effective transmission of the sphere’s potential to the head of the streamer channel.

Insulation Breakdown Characteristics of Inverter Surge Resistant Enameled Wire Prepared with Organic/Inorganic Hybrid Nanocomposite

Insulation breakdown characteristics of an inverter surge resistant enameled wire were investigated in a twisted pair prepared with organic/inorganic hybrid nanocomposite. Organic polymer was polyesterimide-polyamideimide (EI/ AI) and inorganic material was a nano-sized silica. The enamel thickness was 50 μm and the diameters of enameled copper wires were 0.75, 1.024, and 1.09 mm, respectively. There were many air gaps in a twisted pair. Therefore, when the voltage was applied to the twisted pair, enamel erosion took place in the air gap area because of partial discharge according to Paschen's law. The insulation lifetime of the hybrid wire (HW ) was 41,750 sec, which was 515.4 times more than the 81 sec of EI/AIW. In addition, the shape parameter of HW was 2.58, which was 3.4 times higher than 0.75 of EI/AIW.

Effect of direct fluorination on surface partial discharge characteristics of polyimide films

Surface charge accumulation induced by partial discharge has a great effect on breakdown characteristics of polymer insulation. By changing the chemical component in the surface layer of insulation material, fluorination will cause the corresponding change in electrical property. This paper presents a study aimed at clarifying the effect of fluorination time on the partial discharge of fluorinated polyimide (PI) film. PI samples were fluorinated in a reaction kettle at about 328 K (55 °C) using a F <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> blend with volume fraction of 20% nitrogen and 0.05 MPa (500 mbar) for respectively 15, 30, 45, 60 and 90 min. The partial discharge experiment was tested at room temperature with the relative humidity of ~ 40%. A wideband detection system and a video recorder were employed to detect the partial discharge current and record discharge images in a shielding darkroom. Obtained results show that the conductivity of polyimide film becomes large after the fluorination. The corona resistance time increases with the fluorination time increases from 0 to 45 min, while it decreases when the fluorination time increases from 45 to 90 min. Recurrent plot (RP) analysis of discharge current shows that the change of chemical component in surface layer of polyimide film by fluorination makes the partial discharge become more stable and regular.

Breakdown Characteristics of Oil Impregnated Insulation Paper with Different Ageing Condition under the Composite Electric Field

There are many researches about the effect of oil/paper proportion, testing temperature, AC/DC ratio and other effects on the breakdown characteristics of oil-paper insulation under the composite electric field. However, the breakdown characteristics of oil-paper insulation under the composite electric field in the process of ageing are rarely reported. In this paper, the breakdown characteristics of the oil impregnated insulation paper with different ageing condition under the composite electric field were investigated. Results show that there is no necessary link between the breakdown characteristics and the ageing degree of oil-paper insulation. The breakdown voltage value decreased with the rise of the AC component under the composite electric field. The breakdown voltage with aged paper changed by new paper is slightly higher than that of the aged oil being changed by new oil.

Breakdown of Air-Polymer Composite Insulation due to Partial Discharge and Influence of Thermal Insulation

Breakdown characteristics of air-polymer composite insulation under repetition of partial discharge have been studied using needle-plane electrode system. The purpose of this study is to clarify the effect of repetitive partial discharges and thermal properties of polymer on breakdown mechanism of air-polymer composite insulation under the application of square pulse voltage. The experiments were also performed by inserting a thermal insulating film between an electrical insulating film for breakdown and the plane electrode. The number of pulse voltage up to breakdown (NBD) was measured with variation of pulse frequency. With increasing the pulse frequency, the sudden drop of the NBD appeared over a threshold of pulse frequency. The threshold frequency became lower by inserting thermal insulation. The results observed with thermograph show that surface temperature was increased with increasing pulse frequency and increased more remarkably around the threshold frequency. With insertion of the thermal insulating film, the surface temperature rise appeared at lower frequency. These results on the breakdown characteristic and the thermal characteristic were successfully explained based on the numerical analysis of the thermal circuit.

Insulation breakdown characteristics of UHV-class oil-immersed transformer for lightning impulse withstand voltage test waveform - K-factor value and front time related characteristics

The lightning impulse withstand voltage test waveform for electrical power equipment is specified in IEC 60060-1 "High-voltage test techniques". Now that ultrahigh voltage-class (UHV-class) electrical power equipment is being introduced and increasingly used, the electrical capacitance of equipment to be tested has increased, which has eventually hampered the test using the waveform that conforms to the previous standard. To address this issue, the IEC revised the IEC 60060-1 standard in 2010 and now is studying the standard for UHV-class electrical equipment. In this revision, an evaluation method using the k-factor function (test voltage function) was introduced, whereby the overshoot waveform was converted to a test voltage waveform. To date, concerning these k-factor values, measurement results based on experiments have been reported, but most were for small-scale insulation models with breakdown voltage levels mainly around 100 kV. This paper reports the experimental result of insulation breakdown characteristics for lightning impulse withstand voltage test waveforms in the model with the largest oil gap possible assuming an actual UHV-class oil-immersed transformer. Breakdown voltages and breakdown times were measured with the superimposed oscillation frequency, overshoot rate and front time as parameters. Consequently, it was indicated that, as the overshoot rate increased, the 50% breakdown voltage was inclined to increase. Subsequently, the evaluation using the k-factor function is considered to be useful for UHV-class oil immersed transformer. The result of evaluation of the k-factor value based on the experimental result was almost identical to the result of the European Project, which is the basic data of the existing k-factor function. As a result, it emerged that the existing k-factor function itself was most likely to be applicable to UHV-class oil-immersed transformers. Furthermore, since the extension of the front time up to about 3.6 μs caused only a small change in insulation breakdown characteristics, the extension of the front time up to about 3.6 μs might possibly be allowed in the standard that assumes a UHV-class transformer.